Methods and Case Studies of Ultra‐deep Fault Seal Evaluation

Abstract

Fault seals are significant for petroleum exploration and production. This study summarizes the fault sealing impacting factors, including lithological juxtaposition, mud smearing, fault rocks and the fault plane stress states, as well as evaluation methods like Allan maps and Shale Gouge Ratio (SGR). The seal evaluation for a wrench fault focuses on its particular structural features. The evaluation methods were applied to the Jinma-Yazihe structure and the Shunbei oilfield. The source rock is the Xujiahe Formation of the Upper Triassic, the reservoirs and caprocks being of the Shaximiao Formation of the Lower Jurassic. The fault sealing evaluations in major faults proved the reservoir formation processes in the wells Jinfo 1 (JF1) and Chuanya 609 (CY-609), based on the editions of the Allan map showing lithological juxtaposition, the calculation of SGR showing mud smear and analyses of fault stress states. The analyses of stress states were also applied to Shunbei 5 strike-slip fault in the Shunbei area in Tarim Basin. The various sections along the fault were of different mechanical properties, such as compression and extension. Petroleum exploration has demonstrated that the extensional sections are more favorable for oil accumulation than the compressional sections. These evolutionary methods and other understandings will help in analyses of deep fault sealing.

Fault Evolution and Its Effect on the Sealing Ability of Mudstone Cap Rocks

“…Mudstone cap rocks control the spatial distribution [1][2][3][4] and scale [5,6] of oil and gas accumulation in petroliferous basins. However, whether a mudstone cap rock can seal-in oil and gas is not just related to its own development characteristics [7,8], it also depends on the destructive effect and degree of damage exerted via fault evolution [9][10][11][12][13][14]. The greater the degree of damage that faults inflict on the sealing ability of mudstone cap rocks, the smaller the scale of oil and gas accumulation in the reservoir under the mudstone cap rock, and vice versa [15][16][17].…”